Method of coating p-type germanium with antimony, lead or alloys thereof by electrodeposition and product thereof



May 30, 1967 c. H. DE MINJER 3,322,516

- METHOD OF COATING P-TYPE GERMANIUM WITH ANTIMONY, LEAD OR ALLOYSTHEREOF BY ELECTRODEPOSITION AND PRODUCT THEREOF Filed March 5, 1964INVENTOR.

CLARA HDE MINJER gsu-r United States Patent 3,322,516 METHOD OF COATINGp-TYPE GERMANIUM WITH ANTIMONY, L E A D OR ALLOYS THEREOF BYELEC'I'RODEPOSITION AND PRODUCT THEREOF Clara Henderina de Minjer,Emmasingel, Emdhoven,

Netherlands, assignor to North American Philips Company, Inc., New York,N.Y., a corporation of Delaware Filed Mar. 3, 1964, Ser. No. 349,182Claims priority, applicatiglgr gestherlands, Mar. 5, 1963,

2 5 Claims. (Cl. 29-194) The invention relates to a method of coatingp-type germanium with a layer consisting of antimony, lead or alloysthereof by electrodeposition and to the products obtained by thismethod.

It is an object of the invention to deposit a uniform, well-adherentlayer of Sb, Pb or Sb-Pb alloys on a p-type Ge surface in order to makerectifying alloy contacts or alloy-diflusion contacts of uniform depthfor use in known semiconductor devices, such as transistors or diodes.

It is another object of the invention to make a uniform Sb layer on ap-type Ge surface in order to make an ntype zone of uniform thickness bydiffusing the Sb into the underlying Ge, e.g. for use as a base zone ina known p-n-p transistor.

Various methods are known for coating electrically conductive materialby electrodeposition with a layer consisting of antimony, lead or analloy of these two metals. In addition, for coating germanium withantimony a special alkaline reacting bath is known.

However, it has appeared to be impossible with the known methods to coatp-ty-pe germanium sufliciently uniformly with antimony, lead or an alloyof these two metals, as is necessary for providing very smalllowresistance contacts on germanium transistors. The surface of p-typegermanium as a rule is coated only incompletely with the metal.

Various chemical and electrochemical pretreatments of the p-typegermanium were tested without success, for example the chemical etchingby means of a fluoridecontaining bath, the anodic pickling in apotassium hydroxide solution, the anodic connection in the lead bath fora short period of time in which, then, lead is deposited cathodically.Electrolysis at elevated temperature did not give the desired resulteither.

The invention provides a method by which a complete coating of p-typegermanium with the above metals is possible.

According to the invention, one the surface of p-type germanium, beforea layer of antimony, lead or an alloy of Pb-Sb is applied in knownmanner, a thin layer of antimony is deposited, also electrolytically, ina thickness of approximately 0.1-1 at that cathode potential at whichthe saturation current of antimony is reached but at which no gasdevelopment occurs.

For this purpose preferably a bath is used consisting of an aqueoussolution which contains per liter 15-19 g. of potassium antimonyltartrate, 15-52 g. of citric acid and 375-195 g. of sodium citrate, atroom temperature and at a cathode potential of approximately --850 to1000 mv. with respect to a standard hydrogen electrode.

In order to deposit antimony in a compact gray form byelectrodeposition, it was known that for this purpose a particularcritical value of the current density may not be exceeded, sinceotherwise black antimony is formed with a more or less porous structure.When carrying the invention into effect it has appeared that exactly athigh current densities the p-type germanium is entirely coated with thisblack antimony.

The current density i as a function of the cathode potential E turnedout to have a shape as is shown in the ice diagram of the accompanyingdrawing which holds for an antimony bath of the composition:

16 g. of potassium antimonyl tartrate,

82g. of sodium citrate, and

25 g. of citric acid per liter of solution in water and a pH ofapproximately 5.8.

The stated is measured with respect to a standard hydrogen electrode.Proceeding from less negative towards more negative potential,light-gray antimony begins to be deposited at approximately 600 mv. Whenthe cathode potential is further decreased to approximately -850 mv., atwhich the saturation current density of approximately 0.28 a./dm. isbegins to be deposited. If the potential is still further decreased tolower than approximately 950 mv., gas development sets in, in additionto the deposition of black antimony. The point where this gasdevelopment sets in, depends upon the pH of the electrolysis solution;at low pH-values, namely at pH 6, the gas development occurred alreadyabove 9S0 mv. Therefore, activities are preferably carried out atpH-values above 6 since otherwise the margin in the potential at thebeginning of the occurrence of the saturation current and that at whichthe gas development occurs is small.

It has surprisingly been found that with the so-called black antimony acomplete coating of the surface of p-type germanium is possible.Although this deposit has a somewhat porous structure, a layer ofantimony, lead or an alloy of antimony and lead on it appeared to adhereexcellently to it and to coat it completely. This black antimony isdeposited as a flash with a thickness of approximately 0.1-lg. When thelayer was provided in a greater thickness, the porous structure appearedto present no difliculties yet as regards the complete coating by thesubsequently deposited layer.

When a layer of antimony is to be deposited on p-type germanium, thismay be carried out by means of any known electrolyte after the flash ofblack antimony is deposited. Alternatively, the electrolyte used forproviding the flash of black antimony may be chosen for the purpose, inwhich case the cathode potential must be increased by approximately 200to 3'50 rnv. after the flash is deposited. For depositing thicker layersof antimony, however, this flash-bath is less suitable.

When lead or an alloy of lead and antimony must be deposited on thep-type germanium provided with a flash, it is to be recommended todeposit, from the same electrolyte as from which the flash has beendeposited, a thin layer of gray antimony on the black antimony also byincreasing the cathode potential by 200 to 350 mv. before the germaniumis connected as the cathode in the electrolyte chosen for the purpose.

If the potential is set up by means of a potentiostatic arrangement, itmust be avoided that a great voltage drop occurs at the cathode. In thatcase special attention should be paid to the connection of the germaniumto the current supply wire having a contact resistance which is as lowas possible. If this is not the case, a potential diflerence of even 1volt may occur between the current supply wire and the germanium as aresult of which the required value of the cathode potential for thedeposition of black antimony would not be attained. This may be done inany known manner, for example by locally depositing a thin layer ofindium on the germanium which has been cleaned by scouring and providingon it a copper wire with silver paste. For this purpose, instead ofindium also a thin layer of another metal, for example copper, may bedeposited on the germanium.

If no potentiostat is used, it is not necessary to provide contacts ofvery low resistance but the potential may be read and the current may becontrolled so that this reached, black antimony potential has thedesired value by means of a second contact and a high resistancevoltmeter.

In order that the invention may readily be carried into effect, it willnow be described more fully, by way of example, with reference to theensuing specific example.

Strips of germanium having the dimensions 20 x 2 x 0.25 mm. which wereof the p-type by doping by means of indium in a quantity of 3.5 atomsper cm. with a resistivity of approximately 1 ohm cm. were sawed out ofa single crystal in the [UH-orientation, lapped with Carborundum powderand then pickled for 14 seconds in a mixture of 5 ml. of HNO (d.=1.4),10 ml. of HF 50%, and 5 ml. of ethanol.

A thin layer of indium was deposited electrolytically on one of thesides of the strips and a copper wire was connected with silver paste.

The side of the strip to which the copper wire was connected, and thecopper wire itself, were coated with a layer of nitrocellulose lacquer.

The strips were connected cathodically in an aqueous electrolyte whichcontained per liter:

16 g. of potassium antimonyl tartrate,

82 g. of sodium citrate, and

25 g. of citric acid and which was adjusted at a pH=6.5.

A platinum strip was used as the anode. The potential was adjusted via aHaber-Luggin capillary and a saturated calomel electrode by means of apotentiostat. The electrolysis was carried out at 20 C.

The potential was connected at -600 mv. (with respect to a standardhydrogen electrode), then rapidly brought to 850 mv., maintained at thisvalue for one minute, then connected back to -600 mv. and maintained atthis value for one minute also. This is shown in the E-i diagram of thedrawing. The strips were removed from the bath, rinsed, and then a partwas coated electrolytically with lead and the remainder with an alloy oflead and antimony.

The lead bath had the following composition per liter of solution inwater.

100 g. of lead as lead fiuoborate, 80 g. of fiuoboric acid,

g. of boric acid, and

0.5 g. of gelatine.

The alloying bath contained per liter of solution 3 g. of antimony asantimony fiuoborate or as Sb O and 72 g. of tartaric acid in addition tothe above substances.

The anodes consisted of lead. The cathodic current density was 4 a./dm.The bath temperature was maintained at 20 C. The alloy deposit containedapproximately 3% of Sb.

All deposits obtained covered the whole surface of the p type germanumstrips which was not coated by the nitrocellulose lacquer.

What is claimed is:

1. A method of providing a coating selected from the group consisting ofantimony, lead and mutual alloys thereof on p-type germanium comprisingfirst electrolytically depositing on the surface of said p-typegermanrum a layer of a thickness of about 0.11,u of black antimony, saidelectrodeposition being carried out at a cathode potential at which thesaturation current of antimony is reached but at which no gas isevolved, and then electrolytically depositing on said layer of blackantimony an outer layer of a metal selected from the group consisting oflead, gray antimony and mutual alloys of said metal.

2. A method of claim 1, characterized in that for the deposition of thelayer of black antimony a bath is used consisting of an aqueous solutionwhich contains per liter 1519 g. of potassium antimonyl tartrate,

15-52 g. of citric acid, and

37.5- g. of sodium citrate.

3. A method of claim 2, characterized in that the pH of the solution isadjusted at a value above 6.

4. The method of claim 1 wherein subsequent to the deposition of thethin layer of black antimony a thin layer of gray antimony is depositedfrom the same electrolyte by increasing the cathode potential by 200-3505. A semiconductor device prepared by the method of claim 1.

References Cited UNITED STATES PATENTS 2,603,693 7/1952 Kircher.

FOREIGN PATENTS 608,181 11/1960 Canada.

JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. A METHOD OF PROVIDING A COATING SELECTED FROM THE GROUP CONSISTING OF ANTIMONY, LEAD AND MUTUAL ALLOYS THEREOF ON P-TYPE GERMANIUM COMPRISING FIRST ELECTROLYTICALLY DEPOSITING ON THE SURFACE OF SAID P-TYPE GERMANIUM A LAYER OF A THICKNESS OF ABOUT 0.1-1U OF BLACK ANTIMONY, SAID ELECTRODEPOSITION BEING CARRIED OUT AT A CATHODE POTENTIAL AT WHICH THE SATURATION CURRENT OF ANTIMONY IS REACHED BUT AT WHICH NO GAS IS EVOLVED, AND THEN ELECTROLYTICALLY DEPOSITING ON SAID LAYER OF BLACK ANTIMONY AN OUTER LAYER OF A METAL SELECTED FROM THE GROUP CONSISTING OF LEAD, GRAY ANTIMONY AND MUTUAL ALLOYS OF SAID METAL.
 5. A SEMICONDUCTOR DEVICE PREPARED BY THE METHOD OF CLAIM
 1. 